Merge conflict resolution might be time-consuming and lead to defects, compromising development productivity and system quality. Developers might reduce such adverse impacts by avoiding concurrent programming tasks that are more likely to change the same files and cause merge conflicts. As manually predicting such risk is hard, we propose the TAITIr tool, which approximates the set of files changed by a task (task interface) and reports conflict risk whenever there is an intersection between task interfaces. TAITIr uses as input the acceptance tests related to the tasks for predicting file changes, deriving test-based task interfaces. To assess TAITIr’s conflict risk predictions, we measure precision and recall of 6,360 task pairs from 19 Rails projects on GitHub. Our results confirm that the intersection among task interfaces is associated with a higher probability of merge conflict risk. A minimal intersection predicts conflict risk with 0.59 precision and 0.98 recall. We observe that the higher the intersection size, the higher the number of files changed by both tasks. This way, developers might use the intersection size between interfaces as a degree of conflict risk between tasks, choosing a task to work on depending on it. We also find that TAITIr’s predictions outperform predictions based on changed files by similar past tasks. Our analysis derives several other results, considering variations of our notion of an interface in two dimensions: parts of the test code considered for computing interfaces, kinds of files abstracted by the interfaces.